1. Field of the Invention
This invention relates to an improved process for the production of oligomeric glycol esters of dicarboxylic acids, as exemplified by the production of an oligomeric ethylene glycol ester of terephthalic acid. This oligomer or prepolymer is then used to prepare a high molecular weight polyethylene terephthalate composition suitable for fabrication into films, tapes and fibers.
2. The Prior Art
Synthetic linear polyesters of fiber-forming molecular weight, such as polyesters of aromatic dicarboxylic acids, are prepared by processes involving an esterification step or steps, followed by a polymerization step or steps. Esterification is conducted by reacting dicarboxylic acid with a glycol (direct esterification) or by esterifying the aromatic dicarboxylic acid with a monohydric alcohol such as methyl alcohol and then transesterifying the resulting diester of the acid with a glycol (transesterification). For example, in the transesterification process, the dimethyl ester of terephthalic acid is heated with ethylene glycol in the presence of a transesterification catalyst to produce the bis-glycol ester of terephthalic acid, methyl alcohol and excess ethylene glycol being distilled off. This product is then polymerized by a condensation reaction with the elimination of ethylene glycol by heating the product at elevated temperatures under reduced pressures until a high molecular weight product is formed.
In the direct esterification process, a dicarboxylic acid, e.g., terephthalic acid, is condensed with glycol, e.g., ethylene glycol, to form the ester reaction product. The low molecular weight prepolymer or oligomer is then polymerized by heating under reduced pressures to form a high molecular weight polyester product.
Direct esterification is typically conducted in two steps, primary esterification followed by secondary esterification. In the primary esterification stage, dicarboxylic acid, e.g., terephthalic acid, is reacted with a molar excess of glycol, e.g., ethylene glycol to provide a glycol ester having a chain length or degree of polymerization greater than 1, e.g., 2-3, and a degree of esterification such that at least 70 mole percent, typically 85 mole percent or more, of carboxyl end groups are esterified. Typical operating conditions for the primary esterification stage include temperatures ranging from 220.degree. C. to 280.degree. C. and atmospheric or super-atmospheric pressures.
In the secondary esterification stage, the product of primary esterification is reacted, typically at a temperature of between 250.degree. C. to 280.degree. C., and which is higher than the primary esterification temperature, and at atmospheric pressure or super-atmospheric pressure for a time sufficient to increase the degree of polymerization to between about 4 and about 6. The resultant oligomer must have a substantial excess of hydroxyl end groups in order to be capable of further polymerization into a polymer, see for example, U.S. Pat. No. 3,551,386 to Berkau et al. Thus the oligomers typically will have carboxyl end group concentration (CEG) of from about 200 to about 350 microequivalents per gram (.mu.eq/g).
The oligomeric product of secondary esterification is then polymerized in a low polymerization stage characterized by sub-atmospheric pressure in the range of from 10 to 50 Torr and at a temperature of from 270.degree. C. to 290.degree. C., followed by further polymerization in a final stage at sub-atmospheric pressure of from about 0.5 to 2.5 Torr and a temperature of up to about 300.degree. C. An intermediate polymerization step between the low and high polymerization step may also be employed.
The art has found a correlation of strength retention and hydrolytic stability with a low CEG level in synthetic linear polyesters such as polyethylene terephthalate when fibers manufactured from the polyester are exposed to high temperature operating conditions as when such fibers are used in industrial end uses, such as for reinforcing cords on fabrics in rubber articles including pneumatic tires which operate at high temperatures caused by high speeds and heavy loads.
In prior art processes, in order to minimize the CEG level in the final polymer, such as polyethylene terephthalate, the secondary esterification step or stage is typically conducted at a relatively high ethylene glycol/terephthalic acid (EG/TA) mole ratio (MR), e.g., in the order of 1.30 to 1.50/1, in order to provide fewer unreacted carboxyl end groups in the oligomer. But the use of this high mole ratio results in the formation of glycol dimer, e.g., diethylene glycol, (DEG) which is incorporated in the polymer chain and has the effect of reducing the thermal and light stability of fibers and films prepared therefrom which is undesirable in industrial fibers. The presence of glycol dimer in the polymer product also lowers the softening point of the polymer. The lowering of the softening point is likewise undesirable in industrial fibers. Similarly, glycol dimer may be undesirable in textile fibers for various reasons.
Furthermore, excess ethylene glycol which is present in the oligomer and is not removed during the oligomerization reaction must be removed in the downstream final polymerization, which increases the cost of the polymerization reaction. The molar amount of the glycol such as ethylene glycol can be reduced, i.e., to an EG/TA mole ratio of 1.2:1 or lower. But this can result in a decrease in the degree of esterification with the undesirable result of passing unreacted terephthalic acid into the low polymerizer and a decrease in oligomer degree of polymerization unless primary esterifier temperature is increased, which, in turn results in increased DEG.
Thus, with respect to the present state of the art of preparing synthetic linear polyesters such as polyethylene terephthalate, the CEG level of the polymer can be reduced but only at the expense of increased DEG, a lower oligomer DP and/or the use of substantial excesses of glycol.
It is an object of this invention to provide an improved process for reducing either or both of the CEG and glycol dimer, e.g., DEG, concentration in synthetic linear polyesters. A further object of this invention is to provide a process whereby synthetic linear polyesters are obtained having CEG and/or DEG levels lower than has heretofore been possible in a conventional polymerization process. Still further objects of the invention include the provisions of a secondary esterification process stage which can provide an oligomer having an increased degree of polymerization and containing a deceased content of free glycol. Other objects of the invention will become apparent from a full reading and consideration of the following specification.